Protective device for radio receivers



ilnited States Patent PROTECTIVE DEVICE FOR RADIO RECEIVERS Frederick A.Schaner, Fair Lawn, N. J., assignor to Air Associates, Incorporated,Teterboro, N. J., a corporation of New Jersey Application February 11,1954, Serial No. 409,564

Claims. (Cl. 250-20) This invention relates to protective devices forradio receivers which may be used in the same vicinity as high poweredradio transmitting stations. The invention has particular reference tocircuit elements which are used in radio receiving sets to prevent undueheating of input circuit components which receive radio waves from anantenna.

There are many cases where a sensitive receiving set is located close toa high powered transmitting antenna and even if the receiving set is nottuned to the high powered wave there still may be currents and voltagesset up in the input portions of the circuit which may cause permanentdamage. Such a condition exists aboard boats which use many forms oftransmitting and receiving equipment, including radar, position locatingsystems, and other forms of high powered transmitting systems. Smallportable radio receivers must necessarily use moderate size wire in thetuning coils which form part of the input circuit and when suchequipment is subjected to the application of high power radio waves thecoils may be damaged.

In order to protect the input circuits of such radio receivers a devicemust be used which will not materially reduce the radio signal of thedesired frequency and yet must be able to divert or otherwise eliminatehigh powered undesired signals even when the set is not in operation.

One of the objects of this invention is to provide an improvedprotective device for radio receivers which avoids one or more of thedisadvantages and limitations of prior art devices.

Another object of the invention is to provide a protective device whichis ready for operation at any time whether the set is being used or not.

Another object of the invention is to provide a pro tective device whichrequires little space and which may be installed in receiving setsalready manufactured.

Another object of the invention is to provide a protective device whichrequires no battery power for its operation.

The invention includes a low-pass filter current in combination with asensitive relay having a single pole double throw set of switchingcontacts and a rectifier system which permits the relay to be operatedby the radio frequency waves. This combination is inserted in the radioreceiver between the antenna and the input circuit. When a high poweredundesired signal is applied to the antenna the relay is operated and aresistor is connected to the filter circuit to absorb the undesiredsignal.

For a better understanding of the present invention, together with otherand further objects thereof, reference is made to the followingdescription taken in connection with the accompanying drawings.

Fig. 1 is a block diagram showing the relationship of the filter andrelay in connection with the other receiver components.

Fig. 2 is a schematic diagram of connections showing the filter, relayand resistor connected to the input circuit of the radio receiver.

Fig. 3 is a schematic diagram of connections of an alternate system ofconnecting a rectifier element to the relay winding.

Fig. 4 is a schematic diagram of connections of another alternate methodof connecting the relay winding and a rectifier system which employsboth halves of the received power.

Fig. 5 is a schematic diagram of connections indicating a low-passfilter for ultra-high frequencies.

Referring now to Fig. 1 an antenna 10 may include the usual singleconductor aerial and ground or it may comprise a more elaborate array ofantennas and reflectors. A filter circuit 11 is connected to the antennaand is designed to eliminate all those frequencies above the frequenciesused by the radio receiver. A relay circuit 12 is connected between thefilter circuit and a radio receiver circuit 13. The output of the radioreceiver may be connected to a transducer or a recording system (notshown).

The filter section 11 includes a series of inductors 14 and a series ofparallel connected capacitors 15. Such a filter is well known in the artand has been employed for the elimination of undesired signals for manyyears. The relay 12 includes a winding 16 and an armature 17 on which issecured a movable contact 18. Stationary contacts 20 and 21 are part ofthe relay system, contacts 20-48 comprising a normally closedcombination and contacts 21-18 forming a normally open combination. Anon-inductive resistor 22 is connected between contact 21 and the commonground connection 23. The input coil 24 which forms part of the inputcircuit of the radio receiver 13 is connected between the commonconductor 23 and contact 20 and is thereby connected through the filterto the antenna 10. In order to derive a direct current component fromthe radio fr quency wave a rectifier element 25 is connected in seriesbetween the output of the filter circuit and the relay winding 16 and inorder to further eliminate the alternating component of the radiofrequency wave a capacitor 26 is connected across the terminals of therelay winding 16.

The operation of this circuit is as follows: when the radio receiver 13is in operation receiving waves within its desired range and amplitude,the signals pass through the filter, through contacts 18 and 20, to theinput coil 24 and thence to the remainder of the radio receivercircuity. The desired signals are attenuated very slightly by the filtercircuit and only a small amount of power is shunted through therectifier 25 and the winding 16. The circuit constants are adjusted sothat this loss is a very small amount and does not detract from theusefulness of the radio receiver. If now a very strong undesired signalshould be applied to the antenna having a frequency greater than thecut-off frequency of the low-pass filter the filter circuit alone willcause its elimination and no high current surges will reach coil 24. Ifa very powerful transmitted signal from a nearby transmitting station isapplied to antenna 10 within the range of the radio receiver or at afrequency which is less than the range, a few cycles will pass throughcontacts 18 and 20 and the coil 24 but after a very short time intervalthe current through rectifier 25 and winding 16 will attract armature 17transferring contact 13 from contact 20 to contact 21 thereby divertingthe applied power from coil 24 to resistor 22. This resistor is madewith suificient heat capacity to withstand any surge which may bereceived by the antenna. As soon as the undesired signal stops or isreduced to a value which will not harm the input circuit, contacts 18are returned to their original position and the circuit is ready forreceiving desired signals Within the frequency range of the receivingset. It should be noted that reversion to normal operation isaccomplished without any manual operation or resetting device. It shouldalso be noted that this circuit operates regardless of the setting ofthe remainder of the radio receiving 'set.

The circuit shown in 'Fig. 3 is the same as the relay circuit shown inFig. 2 except that a rectifier element 28 is connected in parallelacross the terminals of winding 16 and a resistor 27 is connectedbetween armature 17 and the relay winding. This circuit works in thesame manner as the circuit shown in Fig. 2. i

The circuit shown in Fig. 4 uses a split relay winding comprising coils16A and 16B, the center tap thereof being connected to armature 17 andthe filter circuit. The outer terminals of winding 16 are connectedtogether and to the grounded line 23. This alternate system provides adouble-wave rectification but otherwise is similar to the circuit shownin Fig. 2.

The circuit in Fig. shows an ultra-high low-pass filter using capacitorsconnected in parallel and the self inductance of conductors 32 and 33 asthe inductance which combines with the capacitance of elements 15 toproduce the filtering action. It will be obvious that the lengths ofconductors between the junction points of capacitors 15 is of primeimportance and must be carefully measured to produce the desiredresults.

A low frequency filter of the type shown in Fig. 2 may be used to stopfrequencies within the range of fifty to six hundred megacycles while afilter such as that shown in Fig. 5 may be employed to eliminatefrequencies within the range of 450 to 3500 megacycles.

While there have been described and illustrated specific embodiments ofthe invention, it will be obvious that various changes and modificationsmay be made therein without departing from the field of the inventionwhich should be limited only by the scope of the appended claims.

What is claimed is:

1. A protective device for radio receivers comprising, a pair ofcontacts in series connection between an antenna and an input circuit ofthe radio receiver, a relay having a winding coupled by a commonconductor to the radio receiver and one of said contacts, a relayarmature carrying the contact which is coupled to the input circuit, anda resistor connected between the common conductor and a third relaycontact which is connected to the armature contact when the relay isoperated.

2. A protective device for radio receivers comprising, a filter circuitand a pair of contacts in series connection between an antenna and aninput circuit of the radio receiver, a relay having a winding connectedbetween the conductor to the radio receiver and one of said contacts, arelay armature carrying the contact which is coupled to the filtercircuit, and a resistor connected between said common conductor and athird relay contact which is connected to the armature contact when therelay is operated.

3. A protective device for radio receivers comprising, a filter circuitand a pair of contacts in series connection between an antenna and aninput circuit of the radio receiver, a relay having a winding connectedbetween a common conductor to the radio receiver and one of saidcontacts, a relay armature carrying the contact which is coupled to thefilter circuit, and a resistor connected between said common conductorand a third relay contact whereby the relay is operated and the inputcircuit switched to the resistor whenever a predetermined amount ofreceived energy is collected by the antenna.

4. A protective device for radio receivers comprising, a filter circuitand a pair of contacts in series connection between an antenna and aninput circuit of the radio receiver, said filter circuit adjusted toeliminate received radio waves having frequencies which lie beyond therange of said receiver, a relay having a winding connected between acommon conductor to the radio receiver and one of said contacts, a relayarmature carrying the contact which is coupled to the filter circuit,and a resistor connected between said common conductor and a third relaycontact which is connected to the armature contact when the relay isoperated.

5. A protective device for radio receivers comprising, a filter circuitand a pair of contacts in series connection between an antenna and aninput circuit of the radio receiver, said filter circuit adjusted toeliminate received radio waves having frequencies which lie beyond therange of said receiver, a relay having a winding whic receives currentfrom the filter circuit, a rectifier connected in series with the relaywinding for producing a direct current component therein, an armature onsaid relay which breaks the connection between said contacts when therelay is operated, and a resistor which is connected across the filtercircuit whenever the relay is operated.

6. A protective device for radio receivers comprising, a filter circuitand a pair of normally closed contacts in series connection between anantenna and an input circuit of the radio receiver, said filter circuitadjusted to eliminate received radio waves having frequencies which lieabove the range of said receiver, a relay having an operating windingwhich receives current from the filter circuit, a rectifier connected tosaid winding so that a direct current component is produced thereinwhenever radio frequency power is applied to the winding, an armatureoperated by said winding current which opens said normally closedcontacts, and a resistor which is connected across the filter circuitand a common grounded conductor whenever the relay is operated.

7. A protective device for radio receivers comprising, a filter circuitand a pair of normally closed contacts series connection between anantenna and an input circuit of the radio receiver, said filter circuitadjusted to eliminate received radio waves having frequencies which lieabove the range of said receiver, a relay having an operating windingwhich is coupled to the filter circuit through a rectifier circuit, saidwinding receiving direct current whenever radio frequency power isdelivered by the filter, an armature operated by said winding currentwhich opens the normally closed contacts, and a resistor which isconnected across the filter circuit whenever the relay is operated.

8. A protective device for radio receivers comprising, a filter circuitconnected between a receiver antenna and the input to the receiver, saidfilter adjusted to eliminate received radio waves having frequencieswhich lie beyond the range of said receiver, a relay having an operatingwinding, an armature, and a single pole double throw system of contacts,one pair of which is normally closed, said normally closed contactsconnected in series between the filter and the input to the receiver,said operating winding connected across the output of the filtercircuit, and a resistor which is connected between part of the filtercircuit and a normally open contact on the relay.

9. A protective device for radio receivers comprising, a filter circuitconnected between a receiver antenna and the input to the receiver, saidfilter adjusted to eliminate received radio waves having frequencieswhich lie beyond the range of the receiver, a relay having an operatingwinding, an armature, and a single pole double throw system of contacts,one pair of which is normally closed, said normally closed contactsconnected in series between the filter and the input to the receiver,said operating winding coupled to the output of the filter circuitthrough a rectifier so that a direct current is applied to the operatingwinding whenever the filter circuit transmits a radio wave, and aresistor which is connected between part of the filter circuit and anormally open contact on the relay.

it). A protective device for radio receivers comprising, a filtercircuit connected between a receiver antenna and the input to thereceiver, said filter adjusted to eliminate winding, an armature, and asingle pole double throw References Cited in the file of this patentsystem of contacts, one pair of which is normally closed, UNITED STATESPATENTS said normally closed contacts connected in series between thefilter and the input to the receiver, said operating Wheeler July 1932winding coupled to the output of the filter circuit through 5 118 0398Bmwn 1932 a rectifier so that a direct current is applied to the operat-681460 July 1934 ing winding whenever the filter circuit transmits aradio 2,144,836 Dletnch 24) 1939 wave, a resistor which is connectedbetween part of the 2,296,101 Foster Sept 15, 1942 filter circuit and anormally open contact on the relay, said relay adjusted for operationwhen the filter circuit 10 delivers an amount of radio frequency powerwhich is sufficient to damage the radio receiver.

